Superheat and sub-cooling control of refrigeration system

ABSTRACT

A refrigeration vapor compression system including a compressor, an electronic expansion valve with closed loop feedback into a controller, sensors to measure and monitor the system superheat and sub-cooling, and condenser fans controlling the flow of air through the condensing coils, the refrigeration system is operated in at least one of two functional modes to either give priority control to the level of superheat in the system or maintain a minimum level of sub-cooling in the system.

CROSS REFERENCE TO RELATED PATENTS AND APPLICATIONS

This application claims priority to and the benefit of the filing dateof U.S. Provisional Patent Application Ser. No. 61/873,971, filed Sep.5, 2013, which application is hereby incorporated by reference.

BACKGROUND

The present disclosure broadly pertains to the field of refrigerantvapor compression systems, and particularly to a system for dynamicallyoptimizing capacity and efficiency during less than optimal operatingconditions to provide enhanced performance.

Superheat is typically defined as the amount of heat added to arefrigerant vapor after a change of state occurring in the evaporator ofa refrigeration system. This measurement is an indicator of theperformance of the evaporator portion of the system and can have adirect influence on overall system efficiency. It is known to calculatethe system superheat using a variety of methods, i.e. usingpressure/temperature sensors located between the outlet of theevaporator coil and the compressor(s) or by using two temperaturesensors, one located between the expansion valve and the evaporator coiland the other located between the evaporator and the compressor(s). Toomuch superheat indicates the evaporator coil is only partially floodedand portions of the available heat transfer surface of the evaporatorcoil are not utilized. Too little superheat indicates the evaporatorcoil is over-flooded and liquid refrigerant may be exiting theevaporator and entering the compressor with negative performance andreliability effects on the compressor performance. In a typical system,the expansion valve is designed to ensure an excess of superheat to,among other things, avoid any liquid from returning to the compressor.

It is known to modify the superheat setting based on system performanceparameters. For example, as described in U.S. Pat. No. 8,156,750 toButorac et al. (Butorac), a system superheat level is maintained bydynamically adjusting the individual superheat setting of a plurality ofevaporator coils connected to the system. To achieve this, an electronicexpansion valve is opened or closed during the refrigeration systemoperation to allow either more or less liquid refrigerant to flow intothe evaporator coil to maintain a specified level of superheat foroptimal utilization of the evaporator coil heat exchanger surface areabased on control limits established for the particular system. Thoughthis approach utilizes a dynamic superheat setpoint for the system, thesystem still fails to maximize system performance under certainconditions.

Another performance parameter in vapor compression refrigeration systemsis the concept of sub-cooling, defined as the amount of heat removedfrom the liquid refrigerant after the change of state from high pressurevapor to high pressure liquid in the condensing coil. The amount ofsub-cooling is important to the efficiency and capacity of therefrigeration system and can be problematic particularly when thesub-cooling is too low. Sub-cooling assures that the refrigerant is in asingle phase, liquid state as it is delivered to the expansion valve. Ifthe sub-cooling is too low and a two phase mixture of liquid and vaporis delivered to the expansion valve, the consequences are primarilytwo-fold: 1) the mass flow of the two-phase mixture is reduced ascompared to single phase liquid and system capacity is reduced affectingefficiency, and 2) the two-phase mixture passing through the expansionvalve can create cavitation potentially damaging the seat of theexpansion valve.

In single speed systems where the compressor and condenser fans operateeither on or off, the sub-cooling is controlled by system design andproper refrigerant charge. These single speed systems, due to the lackof adaptive controls, are limited in operation to relatively highoutdoor ambient temperatures and specific evaporator loading. Invariable speed systems, where the compressor is operated at variablerates, the resulting pressures in the system can result inless-than-optimal sub-cooling levels which are also compounded byvariations in the ambient temperature and evaporator load. It is knownin the art to vary the speed of the condenser fan in response to changesin outdoor ambient temperature to help balance the temperature andpressures in the system to maintain proper sub-cooling levels.

BRIEF DESCRIPTION

In variable speed refrigeration systems where modulation of compressors,indoor blowers, condenser fans with active control of superheat andsub-cooling is possible with appropriate sensors and direct feedbackcontrol loops, conditions can develop where full optimization of eachand every control parameter independent of and without regard tointeraction between these control parameters may cause less than optimalperformance of the system. Reduced capacity and degraded efficiency maybe observed in the varying conditions such systems will routinelyencounter in the installed environment in which they are expected tooperate.

In one aspect of the present disclosure, a refrigeration vaporcompression system comprises an electronic expansion valve, a sensor orsensors to measure system superheat, a sensor or sensors to measuresystem sub-cooling and a controller operatively coupled to theelectronic expansion valve, the superheat sensor(s) and the sub-coolingsensor(s). The controller is configured to control at least oneparameter to maintain a desired superheat and/or subcooling setting.

In another aspect, a method of optimizing a refrigeration vaporcompression system comprises sensing superheat and sub-cooling levels ofthe system, determining if the operating conditions as indicated by thesuperheat and sub-cooling levels demand a priority to maintaining thesuperheat level or a priority to maintain sub-cooling levels for optimalsystem performance, and adjusting the superheat level to an appropriatesetting to maintain peak system performance. Additionally, monitoringhow much adjustment of the superheat level to preserve subcooling ispresent can provide insight into system refrigerant charge levels.

In accordance with another aspect, a refrigerant vapor compressionsystem comprises a compressor, first and second heat exchangers fluidlycoupled to the compressor, a variable orifice expansion valve locatedbetween the first and second heat exchangers, at least one sensor forsensing operating conditions of the system related to a system superheatlevel and a system subcooling level, a controller operative to receivesensed data from the at least one sensor and, based at least in partthereon, control at least one system component to optimize systemperformance. The controller is configured to monitor the systemsuperheat level to determine whether the system superheat level iswithin a prescribed range of a system superheat setpoint, monitor thesystem subcooling level to determine whether the system subcooling levelis within a prescribed range of a system subcooling setpoint, anditeratively: adjust at least one system setting to restore the systemsubcooling level to within the prescribed range of the system subcoolingsetpoint while the system superheat level is within its prescribedrange, increment the system superheat setpoint when either the systemsubcooling level or the system superheat level cannot be maintainedwithin their respective prescribed ranges, and adjust at least onesystem setting to achieve the incremented superheat setpoint.

The at least one sensor for sensing operating conditions of the systemrelated to a system superheat level and a system subcooling level caninclude at least one of a refrigerant temperature sensor, refrigerantpressure sensor or an ambient temperature sensor, the controller beingconfigured to receive information from the at least one sensor and usethe information for monitoring the system superheat level or systemsubcooling level. The controller can include a memory containing alook-up table, the look-up table including at least one of a superheatsetpoint value and/or a subcooling setpoint value corresponding to atleast one sensed operating condition, and the controller can beconfigured to determine at least one of the system superheat setpointand/or system subcooling setpoint by looking up a superheat setpointvalue and a subcooling setpoint value using the at least one sensedoperating condition.

The system can further include a condenser fan, and the controller canbe configured to adjust a speed of the condenser fan to maintain thesystem subcooling level within the prescribed range of the systemsubcooling setpoint. The controller can also be configured to optimizethe condenser fan speed by reducing the speed of the condenser fan whenthe system subcooling level exceeds the system subcooling setpoint. Thesystem can also include a variable output compressor, and the controllercan be configured to adjust an output capacity of the compressor tomaintain the system subcooling level within the prescribed range of thesystem subcooling setpoint. The controller can also be configured toadjust the expansion valve to maintain the system superheat level withina prescribed range of the system superheat setpoint.

In accordance with another aspect, a method of optimizing a refrigerantvapor compression system having a compressor, first and second heatexchangers fluidly coupled to the compressor, a variable orificeexpansion valve located between the first and second heat exchangers,and at least one sensor for sensing operating conditions of the systemrelated to a system superheat level and a system subcooling level, themethod comprises monitoring the system superheat level to determinewhether the system superheat level is within a prescribed range of asystem superheat setpoint, monitoring the system subcooling level todetermine whether the system subcooling level is within a prescribedrange of a system subcooling setpoint, and, iteratively: adjusting atleast one system setting to restore the system subcooling level towithin the prescribed range of the system subcooling setpoint while thesystem superheat level is within its prescribed range, incrementing thesystem superheat setpoint when either the system subcooling level or thesystem superheat level cannot be maintained within their respectiveprescribed ranges, and adjusting at least one system setting to achievethe incremented superheat setpoint.

The monitoring of the system superheat or system subcooling can includeusing at least one sensor for sensing system superheat or systemsubcooling. The at least one sensor for sensing system superheat orsystem subcooling can include at least one of a refrigerant temperaturesensor, refrigerant pressure sensor or an ambient temperature sensor.

The method can further include assigning an initial system superheatvalue and system subcooling value at system startup based at least inpart on at least one of an ambient temperature, a refrigerant linetemperature or a refrigerant line pressure sensed by the at least onesensor. The method can also include adjusting a speed of a variablespeed condenser fan to maintain the system subcooling level within theprescribed range of the system subcooling setpoint and/or adjusting anoutput capacity of the compressor to maintain the system subcoolinglevel within the prescribed range of the system subcooling setpoint. Themethod can include adjusting the expansion valve to maintain the systemsuperheat within a prescribed range of the system superheat setpoint.

In accordance with another aspect, an electronic control unit forcontrolling an associated expansion valve of a refrigerant vaporcompression system having a compressor, first and second heat exchangersfluidly coupled to the compressor, a condenser fan, a variable orificeexpansion valve located between the first and second heat exchangers,and at least one sensor for sensing operating conditions of the systemrelated to a system superheat level and a system subcooling level, theelectronic control unit comprises an input for receiving data from theat least one sensor, an output for sending a control signal to at leastone of the condenser fan, the expansion valve, or the compressor, amemory that stores computer-executable instructions, and a processorconfigured to execute the computer-executable instructions to generatethe control signal, the instructions comprising: monitoring the systemsuperheat level to determine whether the system superheat level iswithin a prescribed range of a system superheat setpoint, monitoring thesystem subcooling level to determine whether the system subcooling levelis within a prescribed range of a system subcooling setpoint and,iteratively: adjusting at least one system setting to restore the systemsubcooling level to within the prescribed range of the system subcoolingsetpoint while the system superheat level is within its prescribedrange, incrementing the system superheat setpoint when either the systemsubcooling level or the system superheat level cannot be maintainedwithin their respective prescribed ranges, and adjusting at least onesystem setting to achieve the incremented superheat setpoint.

The processor can be further configured to assign an initial systemsuperheat value and system subcooling value at system startup based atleast in part on at least one of an ambient temperature, a refrigerantline temperature or a refrigerant line pressure sensed by the at leastone sensor. The processor can be further configured to adjust a speed ofthe variable speed condenser fan to maintain the system subcooling levelwithin the prescribed range of the system subcooling setpoint. Theprocessor can be further configured to adjust an output capacity of thecompressor to maintain the system subcooling level within the prescribedrange of the system subcooling setpoint. The processor can also befurther configured to adjust the expansion valve to maintain the systemsuperheat within a prescribed range of the system superheat setpoint.

In accordance with another aspect, a refrigerant vapor compressionsystem comprises a compressor, first and second heat exchangers fluidlycoupled to the compressor, a variable orifice expansion valve locatedbetween the first and second heat exchangers, at least one sensor forsensing operating conditions of the system related to a system superheatlevel and a system subcooling level, and means to receive sensed datafrom the at least one sensor and, based at least in part thereon,control at least one of the compressor or variable orifice expansionvalve to maintain system performance. The means configured to: monitorthe system superheat level to determine whether the system superheatlevel is within a prescribed range of a system superheat setpoint,monitor the system subcooling level to determine whether the systemsubcooling level is within a prescribed range of a system subcoolingsetpoint and, iteratively: adjust at least one system setting to restorethe system subcooling level to within the prescribed range of the systemsubcooling setpoint while the system superheat level is within itsprescribed range, increment the system superheat setpoint when eitherthe system subcooling level or the system superheat level cannot bemaintained within their respective prescribed ranges, and adjust atleast one system setting to achieve the incremented superheat setpoint.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an exemplary system in accordance withthe present disclosure;

FIG. 2 is a block diagram of an exemplary controller in accordance withthe present disclosure; and

FIG. 3 is a flowchart of an exemplary method in accordance with thepresent disclosure.

DETAILED DESCRIPTION

With reference to FIG. 1, a variable capacity refrigeration system isidentified generally by reference numeral 10. It will be appreciatedthat the illustrated system is just one of a variety of refrigerationsystems, and that aspects of the disclosure are applicable to a widevariety of such systems. The system 10 includes a compressor 12, whichcan be a modulating variable speed compressor or a tandem compressorsystem, for example. The compressor 12 is fluidly connected to acondensing coil 14 (heat exchanger) by a compressor refrigerantdischarge tubing 16. The condensing coil 14 is located in closeproximity to a condensing fan assembly 18, which includes a fan bladeand a fan motor. The condensing fan motor can be a fixed speed, multiplespeed or variable speed design. The outlet of the condensing coil 14 isfluidly connected to an inlet of an expansion valve 20 by refrigerantliquid tubing 24. The expansion valve 20 controls the flow ofrefrigerant into an evaporator coil 28 (heat exchanger) and creates apressure drop within the refrigerant tubing. The outlet of theevaporator coil 28 is fluidly connected to an inlet of the compressor 12by refrigerant (vapor) tubing 30, thus completing the exemplary closedrefrigeration circuit.

In operation, the compressor 12 receives refrigerant vapor from theevaporator 28 and mechanically compresses the vapor from a low pressureto a higher pressure. This high pressure vapor travels to the condensingcoil 14 via tubing 16. The condensing fan assembly passes ambient airacross the condensing coil 14, removing heat from the high pressurevapor coming from the compressor 12 and enabling the refrigerant vaporto change state to a high pressure liquid within the condensing coil 14.The high pressure liquid refrigerant is further cooled in the lastportion of the condenser coil 14, then exits the condensing coil 14 intothe liquid line 16 as a sub-cooled liquid. The level of sub-cooling isdefined as the difference of the liquid refrigerant temperature and thesaturation temperature of the refrigerant at the pressure within theliquid line. Typical sub-cooling levels might be approximately 10degrees F. In the illustrated embodiment, the sub-cooling level in theliquid line is measured by a pressure/temperature sensor 35, whereby thesaturation temperature is calculated based on the sensed pressure andsubtracted from the liquid temperature to give the sub-cooling level.

Once the refrigerant exits the condensing coil 14, it moves through therefrigerant liquid tubing 24 to the expansion valve 20. The expansionvalve 20 meters the liquid refrigerant into the evaporator coil 28,creating a pressure drop. The reduction of pressure in the refrigeranttubing causes the liquid refrigerant to begin boiling, absorbing heat asair is passed over the evaporator coil 28 by the indoor blower assembly19. The refrigerant continues to boil and absorb heat in the evaporatorcoil 28 until it becomes a single phase vapor at which point the vaporwill continue to heat above the saturation temperature. This added heatis the superheat of the system and is defined by the difference betweenthe vapor temperature and the saturation temperature correlating to thepressure inside the evaporator tubing. System efficiency is maximized inthis embodiment by maintaining an optimum superheat value (e.g., 5degrees F.) with an electronic expansion valve and superheat controller,which can monitor the superheat level by a pressure/temperature sensor39 mounted in the refrigerant suction line 30. The superheatedrefrigerant vapor continues to the inlet of the compressor 12 where thecycle begins again.

During normal operation, a system controller 41 monitors inputs(superheat, subcooling, compressor speed, ambient temperature, etc.) andcontrols one or more of the system components based on system designparameters. The system controller 41 is illustrated schematically inFIG. 2, and generally includes an input 46 for receiving one or moresignals from various system components, an output 48 for outputting acontrol signal to one or more system components, a processor 50 and amemory 52. Stored within memory 52 are one or more modules includinginstructions used by the processor for performing various controlfunctions. For example, a superheat module 54 and a subcooling module 56are provided for carrying out at least some of the controller functionsdescribed herein. A lookup table (LUT) 58 is also stored in the memory52 and contains various superheat setpoint values and/or subcoolingsetpoint values for various system conditions. For example, the LUT 58may contain superheat and/or subcooling setpoint values corresponding toone or more sensed operating conditions such as ambient temperature,refrigerant line temperature, refrigerant line pressure, etc.

As noted, the controller 41 controls one or more of the systemcomponents based on system design parameters. In some applications, thiscan include adjusting the flow through the variable electronic expansionvalve 20 by adjusting the valve opening to maintain the superheatsetpoint. In one exemplary embodiment, the superheat setpoint can beapproximately 5 degrees F. for optimal system performance. Thecontroller 41 also monitors sub-cooling and is capable of adjusting thecondenser fan 18 speed to maintain the sub-cooling setpoint. In oneexemplary embodiment, the subcooling setpoint can be approximately 10degrees F. or greater. In normal operation, sub-cooling can bemaintained above the target threshold and the controller 41 givespriority to maintaining the superheat level as close to the target levelas possible, optimizing the usage of the evaporator coil 28 (heatexchanger) surface area and, thus, maximizing the capacity andefficiency of the system. Additionally, if return and supply duct airtemperatures (not shown) are monitored, the superheat setpoint can beadjusted to optimize system performance as operating conditions vary.This can be considered a normal mode of operation (e.g., superheatpriority mode).

If during normal operation the system controller 41 senses thesub-cooling level dropping below a defined minimum level (e.g., outsidea prescribed range), for example 6 degrees F., and could not control thecondenser fan 18 (or other sub-cooling control method) to increase thesub-cooling level above the minimum threshold, the controller 41 can beconfigured to change operation modes and enter an alternate modereferred to herein as the sub-cooling priority mode. In the sub-coolingpriority mode, the system controller 41 would adjust (e.g., increment)the superheat level to enhance the sub-cooling capabilities of thesystem under the operating conditions encountered. These conditionsmight include low demand, low ambient temperature, or a combination ofboth. Such conditions might commonly occur during periods of coolerambient temperatures (e.g., night time, winter, etc.)

When operating in the sub-cooling priority mode, the system controller41 would increase the superheat setpoint and begin closing theelectronic expansion valve to obtain the new level. By increasing thesuperheat level, system pressures would increase effectively raising thesaturation temperature in the condensing coil 14 which would increasethe temperature difference between the outside ambient temperature andrefrigerant saturation temperature. Additionally, as the superheat levelis increased the relative amount of liquid refrigerant in the evaporator28 decreases and that liquid refrigerant must then move to the condensercoil 14, enhancing the ability for the heat exchanger to subcool thisliquid. The increased temperature difference and increased amount ofliquid charge in the condenser 14 would effectively result in a highersub-cooling level, insuring the refrigerant in the liquid line wasindeed single phase liquid being delivered to the expansion valve 20.Even though some system performance could be sacrificed by raising thesuperheat level during this mode, overall system performance would beoptimized by maintaining the sub-cooling levels in the proper range.Additionally, by monitoring the amount of dynamic adjustment ofsuperheat setpoint required to obtain desired subcooling levels, achange (reduction) in the total system refrigerant charge level can berecognized and a system diagnostic provided to the operator.

In an alternate configuration, the system can further include an outdoorambient temperature sensor 43 that is monitored by the system controller41. The system controller 41 can use information from the liquid linesensor 35, either pressure, temperature or a combination of both,compare the liquid line data to the outdoor ambient temperature, andswitch to the sub-cooling priority mode based on this comparison. Forexample, if the liquid line temperature was measured to be greater than15 degrees F. above the ambient temperature, this might indicate asituation in which the condenser fan speed must be increased in aneffort to increase the airflow through the condenser to increasesub-cooling performance, or, if adjusting the condenser fan speed provedinadequate to correct the subcooling level, demanding a switch tosub-cooling priority mode and, thus increase the superheat setpoint. Thesystem controller 41 continues to monitor the sub-cooling levels andassociated system performance until conditions are such that bothsub-cooling and superheat can be maintained within the respectivesetpoint targets. This relationship between the liquid line temperatureand outdoor ambient temperature can also be used to indicate othersystem performance issues, such as excessive condenser coil airflow, inwhich case the system power consumption may be greater than necessary,adversely affecting system efficiency. For example, if the liquid linesaturation temperature was measured to be less than 5 degrees F. abovethe ambient temperature, this might indicate excessive condenser airflowin which the condenser fan speed may be reduced to conserve energywithout adversely affecting overall system performance. Additionally, ifa trend towards higher and higher superheat settings for a given set ofoperating conditions is observed, this may indicate a reduced amount ofrefrigerant available to obtain the desired system balance and give theopportunity for the control, recognizing the trend, to alert systemmonitors of a potential leak in the refrigeration system.

Turning to FIG. 3, a flowchart of an exemplary method in accordance withthe present disclosure is illustrated. The method 60 begins with processstep 62 wherein the system initiates a cooling cycle with presetparameters based on initial demand signal. For example, the presetparameters can include a superheat setpoint of 5 degrees F. and asubcooling setpoint of 10 degrees F. In process step 64, the subcoolingof the system is monitored (e.g., using the temperature/pressure sensorsas described above). In process step 66, if the subcooling is withintarget parameters (e.g., between the upper and lower control limits,also referred to as a prescribed range, for example 14 degrees and 10degrees, respectively), the method proceeds to process step 68 andremains in normal mode (e.g., superheat priority mode). In this mode,the system components (e.g. condenser fans, compressor capacity) arecontrolled to maintain the superheat level at a setpoint (e.g., 5degrees F.). This includes, among other things, adjusting the expansionvalve and/or regulating the indoor blower motor.

If the subcooling of the system is not within target setpoint tolerancesin process step 66, the method diverts to process step 70 where thesystem components are controlled to restore the subcooling level to thetarget setpoint. At process step 72, if the subcooling level is restored(e.g., above an upper limit tolerance, greater than 10 degrees F. inthis example), the method returns to process step 68 and the system isoperated in superheat priority mode with the system superheat beingmonitored in process step 74. If the subcooling level is not restored,then in process step 76, if the system parameters have not been adjustedto max/min limits, the method evaluates system operating conditions andadjusts the sub-cooling setpoint as indicated by one or more systeminputs (process step 77), the method loops back to process step 64 andthe method continues to monitor/adjust the subcooling as described tobring it back into desired range. By way of example, system parametersare considered adjusted to max/min limits when no further adjustment canbe made to restore the subcooling level, such as adjusting the condenserfan speed, without disturbing/altering the system superheat setpoint.

Accordingly, if in process step 76 it is determined that the systemparameters have been adjusted to max/min limits, the method proceeds toprocess step 78, where the system enters a subcooling priority mode.This mode gives priority to restoring system subcooling and allowssystem superheat to temporarily deviate from target levels. In processstep 80, the subcooling is monitored, and in process step 82 if thesubcooling level is restored, the method reverts to process step 64 andthe method loops to monitor/control system subcooling.

Otherwise, if at process step 82 the system subcooling is not restored,then the method proceeds to process step 84 where the system superheatsetpoint is incremented. For example, the system superheat setpoint of 5degrees may be adjusted to 7 degrees, then revert back to process step74 to stabilize the superheat at the new setpoint. Once the systemsuperheat is within target tolerances in process step 86, then themethod reverts back to process step 64 and the method repeats.Otherwise, the method proceeds to process step 88 and the systemparameters are adjusted to restore the superheat setpoint. If in processstep 90 the system parameters are adjusted to their max/min limits, thenthe method reverts to process step 64 and the method repeats. If inprocess step 90 the system parameters have not been adjusted to max/minlimits, then the method loops back to process step 74 and until thesuperheat value is restored.

The exemplary embodiment has been described with reference to thepreferred embodiments. Modifications and alterations can occur to othersupon reading and understanding the preceding detailed description. It isintended that the exemplary embodiment be construed as including allsuch modifications and alterations insofar as they come within the scopeof the appended claims or the equivalents thereof.

1. A refrigerant vapor compression system comprising: a compressor;first and second heat exchangers fluidly coupled to the compressor; avariable orifice expansion valve located between the first and secondheat exchangers; at least one sensor for sensing operating conditions ofthe system related to a system superheat level and a system subcoolinglevel; a controller operative to receive sensed data from the at leastone sensor and, based at least in part thereon, control at least onesystem component to optimize system performance, the controllerconfigured to: monitor the system superheat level to determine whetherthe system superheat level is within a prescribed range of a systemsuperheat setpoint; monitor the system subcooling level to determinewhether the system subcooling level is within a prescribed range of asystem subcooling setpoint; and, iteratively: adjust at least one systemsetting to restore the system subcooling level to within the prescribedrange of the system subcooling setpoint while the system superheat levelis within its prescribed range; increment the system superheat setpointwhen either the system subcooling level or the system superheat levelcannot be maintained within their respective prescribed ranges; andadjust at least one system setting to achieve the incremented superheatsetpoint.
 2. The system of claim 1, wherein the at least one sensor forsensing operating conditions of the system related to a system superheatlevel and a system subcooling level includes at least one of arefrigerant temperature sensor, refrigerant pressure sensor or anambient temperature sensor, the controller being configured to receiveinformation from the at least one sensor and use the information formonitoring the system superheat level or system subcooling level.
 3. Thesystem of claim 2, wherein the controller includes a memory containing alook-up table, the look-up table including at least one of a superheatsetpoint value or a subcooling setpoint value corresponding to at leastone sensed operating condition, and wherein the controller is furtherconfigured to determine at least one of the system superheat setpoint orthe system subcooling setpoint by looking up a superheat setpoint valueor a subcooling setpoint value using the at least one sensed operatingcondition.
 4. The system of claim 1, further comprising a condenser fan,wherein the controller is configured to adjust a speed of the condenserfan to maintain the system subcooling level within the prescribed rangeof the system subcooling setpoint.
 5. The system of claim 4, wherein thecontroller is configured to optimize the condenser fan speed by reducingthe speed of the condenser fan when the system subcooling level exceedsthe system subcooling setpoint.
 6. The system of claim 1, furthercomprising a variable output compressor, wherein the controller isconfigured to adjust an output capacity of the compressor to maintainthe system subcooling level within the prescribed range of the systemsubcooling setpoint.
 7. The system of claim 1, wherein the controller isconfigured to adjust the expansion valve to maintain the systemsuperheat level within a prescribed range of the system superheatsetpoint.
 8. A method of optimizing a refrigerant vapor compressionsystem having a compressor, first and second heat exchangers fluidlycoupled to the compressor, a variable orifice expansion valve locatedbetween the first and second heat exchangers, and at least one sensorfor sensing operating conditions of the system related to a systemsuperheat level and a system subcooling level, the method comprising:monitoring the system superheat level to determine whether the systemsuperheat level is within a prescribed range of a system superheatsetpoint; monitoring the system subcooling level to determine whetherthe system subcooling level is within a prescribed range of a systemsubcooling setpoint; and, iteratively: adjusting at least one systemsetting to restore the system subcooling level to within the prescribedrange of the system subcooling setpoint while the system superheat levelis within its prescribed range; incrementing the system superheatsetpoint when either the system subcooling level or the system superheatlevel cannot be maintained within their respective prescribed ranges;and adjusting at least one system setting to achieve the incrementedsuperheat setpoint.
 9. The method of claim 8, wherein the monitoring thesystem superheat or system subcooling includes using at least one sensorfor sensing system superheat or system subcooling.
 10. The method ofclaim 9, wherein the at least one sensor for sensing system superheat orsystem subcooling includes at least one of a refrigerant temperaturesensor, refrigerant pressure sensor or an ambient temperature sensor.11. The method of claim 8, further comprising assigning an initialsystem superheat value and system subcooling value at system startupbased at least in part on at least one of an ambient temperature,refrigerant line temperature or refrigerant line pressure sensed by theat least one sensor.
 12. The method of claim 8, further comprisingadjusting a speed of a variable speed condenser fan to maintain thesystem subcooling level within the prescribed range of the systemsubcooling setpoint.
 13. The method of claim 8, further comprisingadjusting an output capacity of the compressor to maintain the systemsubcooling level within the prescribed range of the system subcoolingsetpoint.
 14. The method of claim 8, further comprising adjusting theexpansion valve to maintain the system superheat within a prescribedrange of the system superheat setpoint.
 15. An electronic control unitfor controlling an associated expansion valve of a refrigerant vaporcompression system having a compressor, first and second heat exchangersfluidly coupled to the compressor, a condenser fan, a variable orificeexpansion valve located between the first and second heat exchangers,and at least one sensor for sensing operating conditions of the systemrelated to a system superheat level and a system subcooling level, theelectronic control unit comprising: an input for receiving data from theat least one sensor; an output for sending a control signal to at leastone of the condenser fan, the expansion valve, or the compressor; amemory that stores computer-executable instructions; and a processorconfigured to execute the computer-executable instructions to generatethe control signal, the instructions comprising: monitoring the systemsuperheat level to determine whether the system superheat level iswithin a prescribed range of a system superheat setpoint; monitoring thesystem subcooling level to determine whether the system subcooling levelis within a prescribed range of a system subcooling setpoint; and,iteratively: adjusting at least one system setting to restore the systemsubcooling level to within the prescribed range of the system subcoolingsetpoint while the system superheat level is within its prescribedrange; incrementing the system superheat setpoint when either the systemsubcooling level or the system superheat level cannot be maintainedwithin their respective prescribed ranges; and adjusting at least onesystem setting to achieve the incremented superheat setpoint.
 16. Theelectronic control unit of claim 15, wherein the processor is furtherconfigured to assign an initial system superheat value and systemsubcooling value at system startup based at least in part on at leastone of an ambient temperature, refrigerant line temperature orrefrigerant line pressure sensed by the at least one sensor.
 17. Theelectronic control unit of claim 15, wherein the processor is furtherconfigured to adjust a speed of the variable speed condenser fan tomaintain the system subcooling level within the prescribed range of thesystem subcooling setpoint.
 18. The electronic control unit of claim 15,wherein the processor is further configured adjust an output capacity ofthe compressor to maintain the system subcooling level within theprescribed range of the system subcooling setpoint.
 19. The electroniccontrol unit of claim 15, wherein the processor is further configured toadjust the expansion valve to maintain the system superheat within aprescribed range of the system superheat setpoint.
 20. A refrigerantvapor compression system comprising: a compressor; first and second heatexchangers fluidly coupled to the compressor; a variable orificeexpansion valve located between the first and second heat exchangers; atleast one sensor for sensing operating conditions of the system relatedto a system superheat level and a system subcooling level; and means toreceive sensed data from the at least one sensor and, based at least inpart thereon, control at least one of the compressor or variable orificeexpansion valve to maintain system performance, the means configured to:monitor the system superheat level to determine whether the systemsuperheat level is within a prescribed range of a system superheatsetpoint; monitor the system subcooling level to determine whether thesystem subcooling level is within a prescribed range of a systemsubcooling setpoint; and, iteratively: adjust at least one systemsetting to restore the system subcooling level to within the prescribedrange of the system subcooling setpoint while the system superheat levelis within its prescribed range; increment the system superheat setpointwhen either the system subcooling level or the system superheat levelcannot be maintained within their respective prescribed ranges; andadjust at least one system setting to achieve the incremented superheatsetpoint.